Field of the Invention:
The invention relates to an apparatus for detecting a geometric relation having a device with a magnetic field source and a configuration formed of a multiplicity of Hall elements, for detecting a geometric position of an object.
In principle, such apparatuses are known, inter alia, from Published European Application No. 0 115 391 A3, corresponding to U.S. Pat. No. 4,737,710, in which a sensor configuration for position detection using four Hall elements that are disposed at a given distance from one another is described. Preferably, in that apparatus, in each case analog output variables of two Hall elements with opposite signs are superimposed on one another so that only a relative change in a magnetic field which acts on the sensor configuration is evaluated. A symmetrical rotating device, which is gearwheel-shaped in the broadest sense, is used as a sensor for a variable magnetic field. A distance between two influences on the magnetic field which are of the same type and are brought about by that device is large in comparison with a distance between the individual Hall elements. The four Hall elements deliver the second derivative of the change in the magnetic field over time. Four Hall elements are used because, as a result, a higher signal deviation is achieved with the same change in magnetic field. The sensor configuration described in Published European Application No. 0 115 391 A3, corresponding to U.S. Pat. No. 4,737,710, serves for relative position detection of a rotating part with the aid of the determination of the difference in magnetic field between two Hall elements in each case.
In many applications for apparatuses for position detection, in particular in the construction of vehicles, it is necessary to precisely identify the actual instantaneous relative positions at all times. That is not possible when using an apparatus according to Published European Application No. 0 115 391 A3, corresponding to U.S. Pat. No. 4,737,710. In the publication "Halbleitersensoren" [Semiconductor Sensors], edited by Prof. Dr. Ing. W. J. Bartz et al., Expert Verlag, in Chapter 6.4 on page 259 et seq., in particular on pages 265 to 267 in Section 6.4.3 entitled "Analoge Positionserfassung" [Analog Position Detection], position detection apparatuses which would fulfill those criteria are described in principle. In that case, the Hall voltage as a function of the distance of a Hall element from a movable permanent magnet is simply used as a distance criterion. However, as can be seen in particular in FIG. 6.20 on page 625, such an apparatus is distinguished by a very low range with a linear relation between the change in distance and the change in the magnetic field. Likewise, it has a strong dependence on the temperature, for example.
The application of such a principle in a throttle valve angle sensor for internal combustion engines is described in German Published, Non-Prosecuted Application DE 38 26 408 A1, corresponding to U.S. Pat. No. 4,893,502. A throttle valve angle sensor for internal combustion engines which is described therein has a permanent magnet that is attached to one end of the throttle valve shaft and produces a constant magnetic flux having a direction which is turned as a function of the rotation of the throttle valve shaft. In that case, a magnetically sensitive element, such as a Hall element, for example, by means of which the change in magnetic flux density is measured as a consequence of the rotation of the permanent magnet, is disposed essentially parallel to the main direction of the outwardly directed magnetic flux of the permanent magnet and at a distance from the permanent magnet. The measured changed in the magnetic flux density is converted in that case into a corresponding change in an electrical signal by means of an amplifying electric circuit.
The output variable of the magnetically sensitive element usually has an equivalent variable which is independent of the magnetic field, a so-called offset, superimposed on it in that case. In the case of an output current it is a superimposed direct current and in the case of an output voltage it is a superimposed direct voltage. That offset and the amplification and the linearity of Hall elements fluctuate very strongly from element to element and with temperature. Furthermore, the linearity range of magnetically sensitive elements, such as Hall elements, for example, is very limited so that with such a previously described, known apparatus, complex adjustment is required and, furthermore, because of the aforesaid problems the resolution, i.e. the dynamic range and the precision of the measurement result of such an apparatus, is not very large. In applications in which the intention is to ensure precise position detection even if brief voltage failures may occur, precision potentiometers are therefore usually used. However, problems occur with the latter due to aging and due to poor contacts resulting from contamination or corrosion.